With regard to possible extraterrestrial life as well as artificial or robotic "life," however, a metric of similarity, while still important, may not be sufficient.
Once developed, the alien idea proved a potent motif for fictional explorations of the singularity or insignificance of humanity cultivated by the extraterrestrial hypothesis.
In the context of extraterrestrial life, even a theory of universal biological evolution does not yet exist, much less a theory of universal cultural evolution.
Poorly graphitized carbon as a new cosmothermometer for primitive extraterrestrial materials.
However, the possibility that extraterrestrial red rain cells could breed on earth introduces a new element to the problem.
Given the imperfect, and sometimes incredible, nature of historical reports, it is also impossible to prove that any single event in the past was extraterrestrial.
Gold, however, does not wish to persuade us of supernatural or extraterrestrial forces.
However, these extraterrestrial invaders of suburbia have breached humanity's ultimate boundary, that between itself and the universe.
